This invention relates to solenoidal field lamps and in particular to methods and configurations for reducing electromagnetic interference generated by these lamps.
A typical solenoidal electric field lamp comprises a single toroidal ferrite core disposed in an ionizable fill gas and electrically connected to a radio frequency energy source by means of windings disposed about the core. While a large frequency range is possible for the operation of the energy source, typical sources operate between the frequency of approximately 50 kilohertz and approximately 5 megahertz. This high frequency energy source produces a constantly varying magnetic field within the ferrite core. The radiative effects of this core magnetic field may be minimized by symmetric placement of the electrical windings about the core. The changing magnetic field within the core, however, according to well established laws of electrodynamics, produces a circular electric field threading through the core. Since the core is disposed in an ionizable medium, a sufficiently strong electric field produces a substantially circular electrical current flowing through the ionizable medium. The resulting circulating currents produce a time varying magnetic dipole field which does produce undesirable radiant electromagnetic energy. The strength of the radiation varies as (d/.lambda.).sup.4 where d is the diameter of the circular current loop and .lambda. is the wavelength at which the high frequency energy source operates. Thus, it is seen that at shorter wavelengths (higher frequencies) the strength of the electromagnetic interference conventionally produced is greater than that at higher wavelengths. Nonetheless, it is desirable to operate such lamps at smaller wavelengths because at these frequencies a much smaller ferrite core is needed and additionally the hysteresis losses in the ferrite are greatly reduced.
The toroid is typically disposed in an ionizable fill gas contained within a translucent envelope internally coated with a phosphor material. The fill gas typically comprises mercury vapor enclosed in a glass, alumina or quartz envelope. The electric discharge current through the fill gas (typically 8 amperes at 5 volts) causes the emission of ultraviolet radiation which is absorbed by the phosphor coating and converted to visible wavelengths. If a purely ultraviolet lamp is desired, then the phosphor coating may be omitted. As used herein and in the appended claims, however, is is to be noted that the term "visible", refers to radiation both in the visible region of the spectrum and in the near visible, ultraviolet, and infrared spectral regions.